This invention relates generally to photomasks and photomask blanks for use in the production of semiconductors, integrated circuits (IC), large-scale integrated circuits (LSI), and like electronic components. More specifically, this invention relates to photomasks of the type generally called hard masks and to their blanks.
In general, a photomask blank is fabricated by forming on an outer surface of a transparent substrate a film of a masking material such as a metal or a material usable instead thereof by a process such as vacuum evaporation or sputtering. By applying a photolithographic process including the steps of forming a photoresist film, pattern exposure to light, and etching to this photomask blank, a thin film of the above mentioned metal or another masking material is left in the form of a pattern such as a pattern of a circuit for an IC or an LSI on the transparent substrate thereby to produce a photomask.
As photomasks known in the art, there have been used so-called hard masks having excellent durability, including chromium masks, low-reflection chromium masks, both-side low-reflection chromium masks, chromium oxide masks, silicon masks, iron oxide masks, and others, as well as emulsion masks using silver emulsion films. More recently, electroconductive masks having electroconductivity even after patternization of masking films thereof are beginning to be used. These electroconductive masks have the advantages of decreased occurrence of pattern deficiencies or deflects caused by charging-discharging of electrostatic charges as well as decreased adherence of dust caused by electrostatic charges; of having the capability of being exposed to an electron beam even when the masking film is non-electroconductive; and also of being utilizable for measurement of dimensions or evaluation of registration of masks by an electron beam system.
However, most of these electroconductive thin films generally have weak resistance to chemicals, and therefore, when washed repeatedly with an acid or an alkali conventionally used for washing of masks, the electroconductive portions will be destroyed. As a result, not only is their function as electroconductive masks lost, but also the film for masking is also destroyed together with the electroconductive film, whereby their function as photomasks is also disadvantageously lost. Further, such an electroconductive film generally has inadequate surface hardness. Consequently, in spite of the use of a hard mask with a high surface hardness, the strength and durability of the entire mask are determined by those of the conductive film, whereby the durability of a photomask to be used particularly for contact printing is poor.
Still another difficultly is that even an electroconductive film, which is itself resistant to chemicals, may sometimes lose its resistance to certain chemicals at the portion at which the electroconductive film is directly contacting the masking film.